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How UV damages eyes

Recently I have received many emails asking for more information about “How UV damages eyes”. Luckily there has been a lot of scientific study on this topic.

However this main point remains true. The only way protect the eyes from harmful UV radiation is by wearing good quality sunglasses from birth.

How UV damages eyes

The human eye is exquisitely sensitive to light (i.e., visible radiant energy). And when the eye is dark-adapted. The retina can detect only a few photons of blue-green light. It is therefore not at all surprising that ocular tissues are also more vulnerable to ultraviolet (UV) and light damage than the skin.

To protect the eyes, humans have evolved with certain anatomical, physiological, and behavioral traits. This does give some degree of protection when we are outdoors during daylight. Especially from the intense overhead solar ultraviolet radiation (UVR). For example, the UV exposure threshold dose for photo-keratitis (“welder’s flash” or “snow blindness”). Would be reached in less than 10 minutes around midday in the summer sun. If measured as falling on a horizontal ground surface.

How UV damages eyes – what parts of the eye are affected

UV exposure can affect three critical ocular structures. The cornea, the lens, and the retina. UV damage to these structures causes eye diseases such as cataracts, pterygium, actinic & droplet keratites, pinguecula and macular degeneration.

UV rays – eye transmission and absorption, all ages

Diagram 1 shows that the cornea transmits radiant energy from 295nm and above. Therefore the cornea absorbs all UV below 295nm. The cornea also absorbs 92% of UV radiation at 300nm and 37% of UV from 340nm to 360nm. All of this absorbed UV in the cornea can cause pterygium, actinic & droplet keratites plus pinguecula.

The aqueous humor then absorbs some UV before reaching the lens. The crystalline lens absorbs almost all incident energy wavelengths of 300nm to nearly 400nm. All of this absorbed UV in the lens causes cataracts plus hardening and yellowing of the lens.

But a very small amount of UV-A does reach the retina in youth. Note the lens becomes more absorbing with age. Thus, there are intraocular filters that effectively filter different parts of the UV spectrum. This then allows in the order of 1% or less to actually reach the retina. Except in children under 9 years. Nevertheless, this small fraction of energy (if photo-toxic) is still of concern.

Finally, oblique rays entering the eye from the temporal side can actually reach the equatorial (germinative) area of the lens.

How UV damages eyes
Diagram 1. UV absorption within an eye

UV rays – eye transmission and absorption, ages under 9 years

Diagram 2 clearly displays that for children under 9 years old between 2 to 5%* of UV rays at 320nm reach the retina. While for older age groups none of these more dangerous UV rays reach the retina. *Note the large variation of 2 to 5% UV rays at 320nm reaching the retina. This is due to the large variation of age in this group. Roughly babies will have 5% of UV rays reaching the retina. While a 9 year old will receive 2%. From 10 years of age this 1 to 2% of UV rays above 340nm still reaching the retina will diminish to 1% by age 30. And less than 1% by 60 to 70 years.

UV + visible light absorption within an eye of children below 9 years
Diagram 2. UV + visible light absorption within an eye of children below 9 years

UV rays – eye transmission and absorption, ages 10 years to young adult

Diagram 3 indicates for people aged 10 until young adulthood that very little UV is reaching the retina. So vision should be good. But around 48% of UV rays from 320nm to 400nm are still entering the lens of the eye. These UV rays reaching the lens of the eye do cause cataracts, hardening and yellowing of the lens. Therefore for adults 40 to 45 years old they do start having problems focusing that leads to prescription eyewear.

UV + visible light absorption within an eye of 10 years to young adult
Diagram 3. UV + visible light absorption within an eye of 10 years to young adult

UV rays – eye transmission and absorption, ages 60 – 70 years

In Diagram 4 (age 60-70) there are clear signs of UV damage and yellowing of the cornea. With the lens now restricting the amount of visible light reaching the retina. Visible light at 400nm has gone from 15% down to 1%. And visible light at 460nm has gone down from 65% down to 40%. This makes seeing in the dark harder. Resulting in the need for really bright lights to read.

UV + visible light absorption within an eye of 60 -70 years adult
Diagram 4. UV + visible light absorption within an eye of 60 -70 years adult

How UV energy is absorbed – the magic of the chromophore

For optical radiation to have an effect on matter. The radiation needs to be absorbed. i.e.the radiant energy needs to be transferred to the material in which the effect is to occur.

Two main mechanisms can be distinguished through which the absorbed radiant energy can take effect:

  • a) Heat: radiant energy is converted into molecular motion (kinetic energy). Such as vibration, rotation and translation. Thus the temperature is increased (photo-thermal effect). Here the radiant energy (measured in Joules, J) absorbed per unit time (s) in a certain volume determines the rise in temperature. i.e. the absorbed radiant power (J/s = Watt, W) per unit volume (m3) or the (specific) absorption rate (W/m3) is the determining factor. (Next to how fast the absorbing volume is cooled by heat exchange with its environment).
  • b) Photo-chemistry: radiant energy can cause excitation of atoms or molecules. Thereby moving the outermost (valence) electrons to higher orbital energy levels. This energy can subsequently be utilized in (photo-) chemical reactions and therefore yielding “photo-products”. The radiation needs to be within a certain wavelength range (the “absorption band”) for the excitation to take place. As the radiant energy is absorbed in discrete quanta, “photons”. Which must match the energy required for the excitation. The (part of the) molecule that absorbs the radiation is dubbed the chromophore.
Chromophores and their absorption bands. (adapted from Jagger 1967)
Chromophores and their absorption bands. (adapted from Jagger 1967)

Of the three types of optical radiation. UV radiation is photo-chemically the most active (the photons carry the highest energy). And it is absorbed by certain common chromophores in organic molecules (e.g. C=O, C=S and aromatic rings; the latter are abundantly present in DNA (Figure 1)). Clearly light is also photo-chemically active in the eye. Visual perception starts with the photo-isomerisation of opsin proteins (in G-protein coupled receptors which trigger the neural signalling).

How UV damages eyes, conclusion

It is an accumulation of small amounts of damage over many years that causes the majority of UV damage to eyes. But you can eliminate your chance of getting UV related eye diseases. Simply by wearing a good quality, close fitting pair of sunglasses with 100% UV protection when outdoors. And yes this does work. I do know of people in their seventies. Who have protected their eyes with sunglasses every-time they are outdoors. And whose eyes show no signs of UV related ageing. Thereby retaining 20/20 vision so they don’t even need prescription eyewear to see.

PS. After reading this post you can understand. To avoid blindness the eyes do everything they possibly can. By absorbing UV rays so the UV does not reach the retina. But this does come at a cost which I have just gone through.

The absorption of UV rays by the chromophores in the vitreous. Does break down the normal jelly like structure into a liquid. This then allows for a “vitreous detachment“. In detail this happens when the vitreous pulls away from the retina. This can cause the retina to tear with possible loss of sight in some people. This does starts to occur in adult’s at 50 years plus. So as someone who wore sunglasses regularly (but not while surfing). I was lucky to have an extra 10+ years before this happened to me. So follow this easy advice.

The only way to avoid UV rays from damaging your eyes when your outside in the sun. Is to simply wear a good quality pair of sunglasses from birth.

References:

  1. European Commission: SCENIHR
  2. Sliney 2002
  3. Jagger 1967

Don’t miss the latest sunglasses “WARNINGS” on this page. They will save your child’s eyesight.

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